Data on blood flow in a human skin relates to various states of the skin such as the state of metabolism activity, the metabolic state, the aging degree, skin cancer, and so on in a skin tissue, and measurement of the blood flow in the skin is important in determining the state of the skin.
The leading cause of death among Japanese is malignant neoplasm, the second leading cause is cardiac disease, and the third leading cause is cerebrovascular disease, and the ratio of death from a vascular lesion is almost the same as that from the malignant neoplasm. In addition, in order to recognize the production and progression of the malignant neoplasm, three-dimensional mapping information about new blood vessels in the periphery and information about the blood flow in the new blood vessels are important. In addition, there is a favorite site of the vascular lesion, and hence temporal and spatial information such as blood flow velocity distribution in the blood vessels is inevitable.
Blood flow measuring methods in recent years include laser Doppler flow velocity measuring method, particle image velocimetry (PIV), micro PIV technique, methods using a ultrasonic blood flowmeter, MRI, CT scan method, and methods on the basis of fusion with numeric simulation. Experiments on biological body include an experiment in the body (In vivo) or an experiment out of the body (In vitro). In general, the PIV measurement which provides information about two-dimensional flow is the in vitro measurement. Since the performances of current MRI and CT scan do not provide sufficient spatial and temporal resolutions, it is difficult to obtain detailed information about the state of a blood circulation using the in vivo measurement.
Accordingly, a measuring method using a laser Doppler system is widely used as a blood flow measuring method because momentary values of blood flow velocities and/or blood flow rate are obtained consecutively in a method noninvasive to the skin. The blood flow measuring method on the basis of the laser Doppler system utilizes monochromaticity and coherency of laser light. If the blood vessel in the skin tissue is irradiated with laser light, the light scattered by blood cells moving in the blood vessel causes a frequency shift by the Doppler effect, whereby the blood flow velocity is obtained on the basis of the amount of the frequency shift.
For example, there is a blood flow measuring device based on the laser Doppler system including a sensor element having a transmitting optical fiber configured to launch laser light onto the skin surface from a light source and a receiving optical fiber configured to receive scattered lights from the skin in pair, both embedded in a probe supporting member which faces the skin. The laser irradiating system on the basis of the laser Doppler method includes a one-beam system which is configured to launch a single beam into a subject and a two-beam system which is configured to split a single beam into two parallel beams, cross the same using a lens or the like, and launch a cross point (point of measurement) into the subject.
In addition, the two-beam measuring method includes a differential method and a reference beam method. The reference beam method splits a single laser beam into two strong and weak beams, and utilizes interference between the light shifted in frequency by the Doppler effect and light scattered by a standstill tissue. As there is a difference in frequency on the order of several hundreds to several tens of Hz between the both scattered lights, the interference between the both lights is detected by a light-receiving element such as a photodiode as electric beat signals, and detected frequency and intensity become signals having values corresponding to the velocity and the number of the blood cells. In addition, the values of these signals are integrated and are converted into signals relating to the flow velocity and the flow rate of the blood flow in the corresponding blood vessel, so that the blood flow rate in the blood vessel is obtained.
In the differential method, a single laser beam is split into two beams, and the two beams are collected and are crossed. At the crossed position, the interference of the scattered lights occurs depending on the directions of irradiation of the laser lights. As the intervals of interference fringes are different depending on the shift amount of the Doppler frequency of the light scattered by the blood cells as particles to be observed, the difference is observed to obtain the blood flow velocity. The differential method has an advantage that the scattered lights can be collected into a wide light-receiving aperture, and hence relatively high signal intensity is obtained.
As disclosed in Patent Document 1, a surface blood flow measuring device provided with a sensor unit having a plurality of sensor elements which detect the surface blood flow of the subject and emit output signals and being brought into abutment with the subject, a signal converting unit configured to converts the output signals from the sensor elements into predetermined measured signals, and a display unit configured to display the measured signals as measured values is also proposed. The surface blood flow measuring device is configured in such a manner that when the sensor unit is brought into abutment with the subject, the output signals are emitted from the respective sensor elements according to the surface blood flow of the subject and are converted into the predetermined measured signals in the output signal converting unit. Since the sensor unit is provided with the plurality of sensor elements, measurement is achieved simultaneously or selectively at a plurality of points in one measurement site. In addition, calibration is also possible by averaging the variations of respective measured values, so that the quantitative measurement is achieved.
As disclosed in Patent Document 2, there is provided a blood flow measuring method on the basis of the laser Doppler system in which light-receiving units are provided at a plurality of positions for at least one light-transmitting unit which launches the laser light onto the subject, the light-receiving units being provided at different distances from the light-transmitting unit, the respective light-receiving units receive scattered lights from the subject, the blood flow rates at the respective light-receiving units are measured, whereby the blood flow rates measured at different depth are recorded simultaneously. In this case, it is preferable to provide a plurality of the light-receiving units at each distance from the light-transmitting unit. It is also preferable to vary the power of the laser light to be launched to the subject. Accordingly, the variations in blood flow rate at each skin tissue can be analyzed, and the blood flow rates in a wide depth range of the skin surface can be recorded at each depth from a shallow portion to a deep portion simultaneously.
Furthermore, as disclosed in Patent Document 3, there is also a blood flow distribution measuring device configured to convert the laser light into sheet light using a cylindrical lens, launch the same via a lens onto a mirror arranged at a focal position of the lens, reflect the same from the mirror in the direction of polarization of the mirror, and measure the blood flow rate two dimensionally.
As disclosed in Non-Patent Documents 1 to 5, laser Doppler current meters which allow multipoint simultaneous measurement using laser sheet light are proposed as a device for measuring the flow velocity of fluid in a conduit or the like other than those in the biometrical bodies are proposed.
Patent Document 1: JP-A-5-15501
Patent Document 2: JP-A-8-182658
Patent Document 3: JP-A-7-100119
Non-Patent Document 1: Collection of Papers from the Society of Instrument and Control Engineers Vol. 39, No. 3 (2003) 218-224
Non-Patent Document 2: Collection of Papers from the Japan Society of Mechanical Engineers (B), Vol. 69, No. 677 (2003-1)25-30
Non-Patent Document 3: Experiments in Fluids 24 (1998) 70-76
Non-Patent Document 4: Experiments in Fluids 36 (2004) 274-281
Non-Patent Document 5: SICE 2002 Aug. 5-7, 2002, Osaka, 2199-2204